bgpd: upon if event, evaluate bnc with matching nexthop

bgpd: upon if event, evaluate bnc with matching nexthop

In BGP, when an interface event is detected or triggered,
the BNC that have a next-hop that matches the interface
are not evaluated.

The paths attached to the bnc context are evaluated in the
following situation:
- In the up event case, if at least one next-hop interface
matched the event interface.
- In the down event case, if there is no alternate next-hop
that does not use the event interface.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>